Electric discharge lasers such as excimer lasers require high voltage power supplies. A prior art typical simplified electric circuit for an excimer laser is shown in FIG. 1. The electric circuit includes a magnetic switch circuit and a power supply for the magnetic switch circuit. A block representing a 1 kV prior art power supply for the laser is shown at 2 in FIG. 1. A more detailed description of the prior art power supply is shown in FIG. 2A and FIG. 2B.
In the typical prior art laser system, the power supply 2 provides high voltage pulses of about 600 volts lasting about 0.2 milliseconds at frequencies such as 1000 Hz. The magnetic switch circuit shown in FIG. 1 compresses and amplifies these pulses to produce electrical discharges across the electrodes of the laser as shown at 4. These discharge pulses across the electrodes are typically about 20,000 volts with duration of about 20 ns. A typical KrF excimer laser with the proper gaseous medium between the electrodes, such as 0.1 percent fluorine, 1.0 percent krypton and about 99 percent neon, the laser produces ultraviolet laser pulses at a wavelength of about 248 nm and an energy-per-pulse of about 10 mJ.
For many applications of excimer lasers such as the light source for integrated circuit lithography, precise control of the laser pulse energy is very important. The laser pulse energy over the typical operating range of the laser is approximately proportional to the discharge voltage which in turn is approximately proportional to the power supply output voltage. Thus, the stability of the output pulse energy is dependent on the stability of the power supply voltage. A great variety of power supply devices are available to supply electric power at controlled currents and voltages to a wide variety of equipment.
Maintaining constant power supply output voltage when the laser is operating continuously at a specific repetition rate, such as 1000 Hz, is a challenge for laser suppliers. This task is made much more difficult when the laser is operated in a pulse mode. A typical pulse mode is one in which the laser is required to produce bursts of about 50 pulses at a rate of 1000 KHz during the bursts with the bursts being separated by a "dead time" of a fraction of a second to a few seconds. When operating in a continuous mode, the best prior art output voltage variation is in the range of about 0.6 percent (about 3 to 3.5 volts). When operating in the pulse mode, the variation during the first few pulses is about 2.5 percent (about 12 to 15 volts).
In a typical lithography excimer laser, a feedback control system measures the output laser pulse energy, determines the degree of deviation from a desired pulse energy, and then sends a signal to a controller to adjust the power supply voltage so that energy of subsequent pulses are closer to desired energy. In prior art systems, this feedback signal is an analog signal and it is subject to noise produced by the laser environment. This noise can result in erroneous power supply voltages being provided and can in turn result in increased variation in the output laser pulse energy.
At many self service gasoline stations, the pump shuts down when the quantity of gasoline purchased has been pumped. A well known technique used at these stations is to slow down the gasoline fill rate when the quantity pumped approaches to within a small quantity of the amount purchased.
What is needed is a better fast finely regulated power supply for equipment such as excimer lasers requiring a fast, precise pulse power source.